1. Field of the Invention
The present invention relates generally to gas turbine engines and, more particularly, to a dual inner central drive shaft for transmitting increased torque from the low pressure turbine to the forward fan without increasing the diameter size of the inner central drive shaft.
2. Description of the Prior Art
Gas turbine engines typically include a core engine having a compressor for compressing air entering the core engine, a combustor where fuel is mixed with the compressed air and then burned to create a high energy gas stream, and a first or high pressure turbine which extracts energy from the gas stream to drive the compressor. In aircraft turbofan engines, a second turbine or low pressure turbine located downstream from the core engine extracts more energy from the gas stream for driving a forward fan. The forward fan provides the main propulsive thrust generated by the engine.
More particularly, in the turbofan engine, the energy from the core engine drives the low pressure turbine, and the latter, in turn, drives an inner central drive shaft of a given diameter size which runs forwardly through the center of the core engine. The inner central drive shaft drives the forward fan to produce thrust. The inner central drive shaft must carry or transmit all of the torque that is necessary to drive the fan.
As the bypass ratios of turbofan engines increase (which means that a greater proportion of the air flow is routed externally of the core engine for producing thrust as compared to internally through the core engine where it is mixed with fuel and ignited for producing gas energy), the core engine in effect becomes smaller. Also, with increased bypass ratio, more torque is required to drive the forward fan.
The conventional solution would be to make the inner central drive shaft larger in diameter in order to accommodate more torque. However, this approach would require a complete redesign of the core engine (if using existing engines) which adds weight and is extremely costly, and an increase in the size of bearings which reduces their useful lives. For a new engine, larger bore sizes add much weight to the core and has the same adverse effect on bearings.
Consequently, a need still exists for an alternative solution to providing more torque to drive the forward fan.